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The official documentation can be found here.

why we need the eval command in redis, if redis is single-threaded?

One way to execute commands in REDIS, is via the EVAL script.

Redis uses the same Lua interpreter to run all the commands. Also
Redis guarantees that a script is executed in an atomic way: no other
script or Redis command will be executed while a script is being
executed.

Since redis is single threaded, why do we need EVAL to offer atomicity? I would expect that this is implied by the one running thread.

Am I missing something? Apologies if my question is pretty simple, I am quite new to redis

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Answers


  1. 0

    Redis uses a single thread to execute commands from many different clients. So if you want a group of commands from one client to be executed in sequence, you need a way to direct Redis to do that. That’s what EVAL is for. Without it, Redis could interleave the execution of commands from other clients in with yours.

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  2. 0

    Every (data path) command in Redis is indeed atomic. EVAL allows you to compose an “atomic” command with a script that can include many Redis commands, not to mention control structures and some other utilities that are helpful to implement server-side logic. To achieve the similar “atomicity” of multiple commands you can also use MULTI/EXEC blocks (i.e. transactions) by the way.

    Without an EVAL or a MULTI/EXEC block, your commands will run one after another, but other clients’ commands may interleave between them. Using a script or transaction eliminates that.

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